Aromatic polycarbonate, generally produced by a phosgene process or a melt process, has shortcomings such as coloration and reduction of molecular weight at the time of melt molding with heating, and addition of heat stabilizers has been known to inhibit these phenomena. As heat stabilizers, various phosphorous acid triesters, epoxy compounds and hindered phenolic compounds are used. The phosphorous triester compounds include, for example, tris(nonylphenyl) phosphite and tris(2,4-di-t-butylphenyl) phosphite. Furthermore, tetrakis(2,4-di-t-butylphenyl) 4,4'-biphenylenephosphonite is similarly used. The amount of heat stabilizer added is usually 0.02-1 part by weight per 100 parts by weight of polycarbonate.
Addition of phosphorous acid diesters is also known.
For example, compositions of aromatic polycarbonate and phosphorous acid diester are reported in Japanese Patent Kokoku No. Sho 37-13775 and preferred amount of phosphorous acid diester added is said to be 0.02-5 parts by weight per 100 parts by weight of aromatic polycarbonate.
Moreover, compositions of aromatic polycarbonate and phosphorous acid diester are reported in Japanese Patent Kokai No. Sho 47-12993. In this case, a chlorine atom containing aromatic polycarbonate is obtained by polymerization. A phosgene process is used, and preferred amount of phosphorous acid diester added to the polycarbonate is said to be 0.01-2.0 parts by weight per 100 parts by weight of polycarbonate.
However, although these phosphorous acid ester type heat stabilizers have a conspicuous effect on heat stability for a short period in melt processing, they still result in deficiencies, such as coloration and adverse effects on recycling molding, hydrolysis resistance of molded articles to hot water and steam and coloration of polymer at heat aging for a long period. These adverse effects depend on the amount of heat stabilizer added and, with an increase in such amount, hot water resistance deteriorates and the degree of polymer coloration increases.
Furthermore, Japanese Patent Kokai No. Sho 62-235357 discloses a process for improving hydrolysis resistance. According to this the amount of phosphorus compound added depends on the amount of chloride remaining in polymer. Also, the amount of remaining alkali metal or alkaline earth metal depends on the amount of phosphorus compound.
In the examples, 0.0037-0.0048 part by weight of chlorine atoms remained in 100 parts by weight of polycarbonate obtained by phosgene process, and from 0.0040 to 0.012 part by weight of organic phosphine or phosphorous acid triester as a phosphorus compound, is added thereto.
This patent publication indicates that hydrolysis resistance is improved by the process. However, a trace trial conducted by using the same blending amounts revealed that improvement in heat resistance is inadequate, and coloration is serious when recycling molding is carried out.
Moreover, Japanese Patent Kokai No. Sho 58-89648 can be referred to as a process of adding phosphorus compound.
This patent publication discloses a composition comprising polycarbonate, to which an organic phosphorous acid ester and an organometallic compound are added, and states that, when organic phosphorous acid ester or organometallic compound is singly added to polycarbonate, superior heat stability and hydrolysis resistance cannot be obtained.
In the examples thereof, from 0.04 to 0.05 part by weight of phosphorous acid triester and from 0.01 to 0.05 part by weight of organometallic compound were added to 100 parts by weight of polycarbonate obtained by phosgene process.
In comparative examples thereof, organic phosphorous acid ester was added alone and a large amount, namely, from 0.04 to 0.05 part by weight, of a phosphorous acid triester was added to 100 parts by weight of polycarbonate obtained by polymerization according to a phosgene process.
The patent publication mentions, that according to this process, retention of molecular weight in heat resistance test and in a hot water resistance test is good, but testing with the same blending amount revealed that molded pellets were colored and coloration due to recycling molding was great.
As mentioned above, polycarbonate prepared industrially by a phosgene process contains at least 0.005 part by weight of chlorine per 100 parts by weight of polycarbonate (Japanese Patent Kokoku No. Sho 59-22743). Various stabilizers have been added to this polycarbonate containing chlorine. However, there have not yet been obtained any stabilized compositions which are free from all of the noted problems: heat resistance, hot water resistance and coloration in recycling molding.
On the other hand, it is difficult to produce, on an industrial scale by a melt process, aromatic polycarbonates which are colorless and transparent, and particularly with out sacrificing strength in properties. ("KOBUNSHI (Polymer)", Vol. 27, Page 521, 1978). Hitherto, it has been difficult to obtain colorless and transparent polycarbonates by a melt process.
Even when stabilizers are added to these polycarbonates, improvement in heat resistance and hot water resistance has not been sufficient.
Thus, no polycarbonates have been known which satisfy all of heat resistance, hot water resistance and recycling moldability.